Hide error when task callback is null

Question

I have 2 tasks that must be performed sequentially and produce an error silently so as not to stop the clock. I did one task, it depends on another, and the first task is to have a callback that is called when it ends with an argument to indicate whether it succeeds. Now my problem is that when calling call with a non-zero argument indicating an error, an msg error occurs, which really causes annoyance, since it distracts me from the actual error that occurred, so there is a way to hide this error message.

Let me show you what I have.

gulp.task('stylus', function (cb) {
    var err = null;
    gulp.src('app/**/*.styl')
        .pipe(plumber(function(error){
            err = true;
            gutil.log(error.toString());
        }))
        .pipe(changed('build/css/',{ extension: '.css' }))
        .pipe(stylus())
        .pipe(gulp.dest('build/css/'))
        .on('end',function(){cb(err)});
});
gulp.task('css-concat',['stylus'], function(){  // depends on stylus task
    gulp.src('build/css/**/*.css')
       .pipe(plumber())
       .pipe(concat('app.css'))
       .pipe(gulp.dest('build/'));
});

gulp.task('watch', ['css-concat'], function () { // run css concat first which in trun runs stylus first

    var cssWatch = gulp.watch('app/**/*.styl', ['css-concat']); // watch the same files in our scripts task


    cssWatch.on('change', function (event) {
        if (event.type === 'deleted') {                   // if a file is deleted, delete it from build
            var t = event.path.replace(".styl",".css").replace(/^.+\\app\\/,"build/css/");
           del(t);
        }

    });
}); 

, css-concat stylus , stylus , change call css-concat.

, , stylus ( - , )

[03:14:38] Starting 'stylus'...
[03:14:39] ParseError in plugin 'gulp-stylus'
Message:
    D:\angularjs\app\app.styl:1:19
   1| my-color = blue.............
------------------------^
   2| 
   3| body
   4|   background-color my-color

illegal unary "...", missing left-hand operand

[03:14:39] 'stylus' errored after 14 ms
[03:14:39] Error: true
    at formatError (C:\Users\Karim\AppData\Roaming\npm\node_modules\gulp\bin\gulp.js:161:10)
    at Gulp. (C:\Users\Karim\AppData\Roaming\npm\node_modules\gulp\bin\gulp.js:187:15)
    at Gulp.emit (events.js:95:17)
    at Gulp.Orchestrator._emitTaskDone (D:\angularjs\node_modules\gulp\node_modules\orchestrator\index.js:264:8)
    at D:\angularjs\node_modules\gulp\node_modules\orchestrator\index.js:275:23
    at finish (D:\angularjs\node_modules\gulp\node_modules\orchestrator\lib\runTask.js:21:8)
    at cb (D:\angularjs\node_modules\gulp\node_modules\orchestrator\lib\runTask.js:29:3)
    at DestroyableTransform. (D:\angularjs\gulpfile.js:28:30)
    at DestroyableTransform.emit (events.js:117:20)
    at D:\angularjs\node_modules\gulp\node_modules\vinyl-fs\node_modules\through2\node_modules\readable-stream\lib\_stream_readable.js:965:16